__________ is a special constituent of chlorophyll without which photosynthesis is not possible.

ANS:B - Nitrogen

Nitrogen is a chemical element with the symbol N and atomic number 7. It is a non-metallic element and is a part of the group 15 of the periodic table. Nitrogen is the most abundant gas in Earth's atmosphere, comprising approximately 78% of the air we breathe. It exists in the atmosphere primarily in the form of diatomic nitrogen molecules (N2). Some key points about nitrogen include:

1. Abundance: Nitrogen is highly abundant on Earth, constituting a significant portion of the atmosphere. However, it is relatively inert in its diatomic form, which means it does not readily react with other elements.
2. Role in Living Organisms: Nitrogen is an essential element for life. It is a key component of proteins, nucleic acids (DNA and RNA), and many other organic molecules essential for the structure and function of living organisms.
3. Nitrogen Fixation: While nitrogen is abundant in the atmosphere, most organisms cannot utilize atmospheric nitrogen directly. Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia (NH3) or other nitrogen compounds through a process called nitrogen fixation. These nitrogen compounds can then be utilized by plants and other organisms in the form of nutrients.
4. Fertilizers: Nitrogen compounds are crucial components of fertilizers used in agriculture to enhance plant growth. Ammonium nitrate, urea, and ammonium sulfate are common nitrogen-containing fertilizers.
5. Industrial Applications: Nitrogen has various industrial applications. It is used in the production of ammonia, which is a precursor to many nitrogen-containing compounds, including fertilizers, explosives, and various chemicals. Nitrogen gas is also used in modified atmosphere packaging to preserve perishable foods.